Photolithography techniques have progressed to the point that the critical dimension (CD) is often smaller than the actinic wavelength of the radiation (e.g., ultraviolet light) employed, requiring the effects of optical diffraction to not only be accounted for, but often utilized for imaging today's sub-micron features. In response, various phase-shifting techniques have been employed to mitigate the detrimental effects of diffraction by taking advantage of the destructive interference caused by phase shifting.
Photomasks incorporating rim-shifting elements are one type of phase-shifting device so employed. Some rim-shifting photomasks employ a pattern of opaque light blocking elements on a transparent substrate. A transparent phase shift material, such as quartz, covers the opaque light blocking elements and the transparent substrate, such that light passing through the thicker transparent phase shift material along the side walls of the opaque light blocking elements is phase shifted with respect to light passing through the thinner phase shift material between the opaque light blocking elements. The intersection of the phase-shifted light from the two regions forms a null on the wafer. This utilizes the effects of diffraction along the edges of the opaque light blockers and produces a sharpened image. The light transmission produced by such phase-shifting is binary, in that two transmissions are produced: a first passing through the thinner phase shift material and a second passing through the thicker phase shift material along the side walls of the opaque light blocking elements. Typically, the second transmission is phase-shifted 180° with respect to the first transmission.